1. Technical Field
This invention relates generally to laser projection systems, and more particularly to a laser-based projection system employing active polarization switches to reduce speckle perceived by a viewer.
2. Background Art
Laser projection devices facilitate the production of brilliant images created with vibrant colors. Laser projection systems are generally brighter, sharper, and have a larger depth of focus than do conventional projection systems. Further, the advent of semiconductor lasers and laser diodes allows laser projection systems to be designed as compact projection systems that can be manufactured at a reasonable cost. These systems consume small amounts of power yet deliver bright, complex images.
One practical drawback associated with using lasers in projection systems is the image artifact known as “speckle.” Speckle occurs when a coherent light source is projected onto a randomly diffusing projection surface. Laser light is highly coherent. Accordingly, when it reflects off a rough surface, components of the light combine with other components to form patches of higher intensity light and lower intensity light. In a detector with a finite aperture such as a human eye, these varied patches of intensity appear as “speckles,” meaning that some small portions of the image look brighter than other small portions. This spot-to-spot intensity difference can vary depending on observer's position, which makes the speckles appear to change in time when the observer moves.
Turning now to FIG. 1, illustrated therein is a prior art system 100 in which an observer 102 may perceive speckle. Specifically, a coherent light source 101, such as a semiconductor-type or standard laser, delivers a coherent beam 104 to a modulation and scanning device 103. The modulation and scanning device 103 modulates the coherent beam 104 into a modulated and scanned coherent beam 105 to form an image on a projection medium, shown illustratively in FIG. 1 as a projection screen 107.
The surface of the projection screen 107 has a random roughness, i.e., as it includes tiny bumps and crevices that are randomly distributed. The reflected light 108 therefore includes portions that combine and other portions that cancel. As a result, the observer 102 views an image 106 that appears to be “speckled.” The presence of speckle often tends to perceptibly degrade the quality of the image produced using the laser projection system.
There is thus a need for an improved speckle-reducing system for use with laser-based projection systems.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.